database update query
Updates query optimization statistics on a table or indexed view. By default, the query optimizer already updates statistics as necessary to improve the query plan; in some cases you can improve query performance by using UPDATE STATISTICS or the stored procedure sp_updatestats to update statistics more frequently than the default updates.
Updating statistics ensures that queries compile with up-to-date statistics. However, updating statistics causes queries to recompile. We recommend not updating statistics too frequently because there is a performance tradeoff between improving query plans and the time it takes to recompile queries. The specific tradeoffs depend on your application. UPDATE STATISTICS can use tempdb to sort the sample of rows for building statistics.
Transact-SQL Syntax Conventions
Syntax
- Syntax for SQL Server and Azure SQL Database UPDATE STATISTICS table_or_indexed_view_name [ { { index_or_statistics__name } | ( { index_or_statistics_name } [...n ] ) } ] [ WITH [ FULLSCAN [ [, ] PERSIST_SAMPLE_PERCENT = { ON | OFF } ] | SAMPLE number { PERCENT | ROWS } [ [, ] PERSIST_SAMPLE_PERCENT = { ON | OFF } ] | RESAMPLE [ ON PARTITIONS ( {
| } [, …n] ) ] | [...n ] ] [ [, ] [ ALL | COLUMNS | INDEX ] [ [, ] NORECOMPUTE ] [ [, ] INCREMENTAL = { ON | OFF } ] ] ; ::= [ STATS_STREAM = stats_stream ] [ ROWCOUNT = numeric_constant ] [ PAGECOUNT = numeric_contant ]
- Syntax for Azure SQL Data Warehouse and Parallel Data Warehouse UPDATE STATISTICS schema_name . ] table_name [ ( { statistics_name | index_name } ) ] [ WITH { FULLSCAN | SAMPLE number PERCENT | RESAMPLE } ] [;]
Arguments
table_or_indexed_view_name
Is the name of the table or indexed view that contains the statistics object.
index_or_statistics_name
Is the name of the index to update statistics on or name of the statistics to update. If index_or_statistics_name is not specified, the query optimizer updates all statistics for the table or indexed view. This includes statistics created using the CREATE STATISTICS statement, single-column statistics created when AUTO_CREATE_STATISTICS is on, and statistics created for indexes.
For more information about AUTO_CREATE_STATISTICS, see ALTER DATABASE SET Options (Transact-SQL). To view all indexes for a table or view, you can use sp_helpindex.
FULLSCAN
Compute statistics by scanning all rows in the table or indexed view. FULLSCAN and SAMPLE 100 PERCENT have the same results. FULLSCAN cannot be used with the SAMPLE option.
SAMPLE number { PERCENT | ROWS }
Specifies the approximate percentage or number of rows in the table or indexed view for the query optimizer to use when it updates statistics. For PERCENT, number can be from 0 through 100 and for ROWS, number can be from 0 to the total number of rows. The actual percentage or number of rows the query optimizer samples might not match the percentage or number specified. For example, the query optimizer scans all rows on a data page.
SAMPLE is useful for special cases in which the query plan, based on default sampling, is not optimal. In most situations, it is not necessary to specify SAMPLE because the query optimizer uses sampling and determines the statistically significant sample size by default, as required to create high-quality query plans.
Starting with SQL Server 2016, sampling of data to build statistics is done in parallel, when using compatibility level 130, to improve the performance of statistics collection. The query optimizer will use parallel sample statistics, whenever a table size exceeds a certain threshold.
SAMPLE cannot be used with the FULLSCAN option. When neither SAMPLE nor FULLSCAN is specified, the query optimizer uses sampled data and computes the sample size by default.
We recommend against specifying 0 PERCENT or 0 ROWS. When 0 PERCENT or ROWS is specified, the statistics object is updated but does not contain statistics data.
For most workloads, a full scan is not required, and default sampling is adequate.
However, certain workloads that are sensitive to widely varying data distributions may require an increased sample size, or even a full scan.
For more information, see the CSS SQL Escalation Services blog.
RESAMPLE
Update each statistic using its most recent sample rate.
Using RESAMPLE can result in a full-table scan. For example, statistics for indexes use a full-table scan for their sample rate. When none of the sample options (SAMPLE, FULLSCAN, RESAMPLE) are specified, the query optimizer samples the data and computes the sample size by default.
PERSIST_SAMPLE_PERCENT = { ON | OFF }
When ON, the statistics will retain the set sampling percentage for subsequent updates that do not explicitly specify a sampling percentage. When OFF, statistics sampling percentage will get reset to default sampling in subsequent updates that do not explicitly specify a sampling percentage. The default is OFF.
Note
If AUTO_UPDATE_STATISTICS is executed, it uses the persisted sampling percentage if available, or use default sampling percentage if not. RESAMPLE behavior is not affected by this option.
Applies to: SQL Server 2016 SP1 CU4.
ON PARTITIONS ( {
| } [, …n] ) ] Forces the leaf-level statistics covering the partitions specified in the ON PARTITIONS clause to be recomputed, and then merged to build the global statistics. WITH RESAMPLE is required because partition statistics built with different sample rates cannot be merged together.